Method of and apparatus for lifting pre-formed slabs



March 21, 1961 l.. H. LEONARD METHOD OF AND APPARATUS FOR LIFTING PRE-FORMED SLABS I N VEN TOR.

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ATTORNEYS March 2l, 1961 L.. H. LEONARD 2,975,560

METHOD oF AND APPARATUS FOR LIFTING PRE-FORMED sLABs Filed sept. 22, 1955 5 Sheets-Sheet 2 Il H @if mmf. 161 Q7 :i u A if All) H Mill AAA A Y j@ E? f i g4/ INVENTOR.

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ATTOR NEYS.

L. H. LEONARD March 21, 1961 METHOD OF AND APPARATUS FOR LIFTING PRE-FORMED SLABS Filed Sept. 22, 1955 5 Sheets-Sheet 3 figg/0.

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BY 6K@ 767400 vana/) METHOD F AND APPARATUS FR LIFTING PRE-PURMED SLABS Lloyd H. Leonard, Inglewood, Caiif. (14877 Valley Vista Blvd., Sherman (laks, Calif.)

Filed Sept. 22, 1955, Ser. No. 535,910

la Claims. (Cl. Sti-Sti) This invention relates to building construction, and `more particularly to a method of and apparatus for erecting buildings, by raising into the desired position preformed door slabs which have been constructed at ground level.

One object of the invention is to provide an improved method of and apparatus for lifting a pre-formed slab by means of a pa-ir of hydraulic jacks supported on each ofthe the usual fixed columns, yand operating alternately.

Another object is to devise improved means for indicating, at a point adjacent each column, any variation in the relative vertical movement of the dierent parts of the slab, so that the slab may be maintained in a plane parallel with its original position.

A still further object of the invention is to provide means whereby, after the uppermost slab has been lifted to the desired elevated position, it may be tilted at an angle to the horizontal to constitute a sloping roof.

Yet another and important object is to devise improved, power-driven hoisting mechanism supported by each coL umnfand so arranged that all of said hoisting mechanisms may be simultaneously operated, and the slab automatically and continuously lifted to its final position, without `any manual operation. ln this connection, means are provided for automatically maintaining the slab substantially level at all times so as to prevent any canting thereof, with its attendant disadvantages and hazards.

With the above and other objects in View, and to improve generally on the details of such apparatus, the invention consists in the construction and combination of parts as hereinafter described and claimed, and illustrated in the accompaning drawings, forming part of this specication, and in which:

Fig. 1 is a schematic view, in the nature of side elevation with parts in section, showing `one form of my improved lifting apparatus;

Figs. 2, 3, and 4 are diagrammatic views illustrating three different forms of slab and showing how my improved level indicating device may be located on each slab;

Fig. 5 is an elevational view similar to Fig. 1 but showing how two slabs may be offset laterially with respect to each other;

Fig. 6 is'a perspective view showing the preferred form of one type of lifting apparatus;

Fig. 7 is a fragmentary sectional elevation on au enlarged scale showing my improved apparatus for tilting a slab to provide la sloping roof;

Fig. 8 is a diagrammatic plan view of such a roof slab showing the preferredlocation of my improved level indicatingmeans; l

Fig. 9 is a diagrammatic side elevation showing a slop- `ping roof slab; and

Fig. 10 is a diagrammatic view illustrating a powerdriven arrangement for automatically lifting a slab to the desired height and for automatically maintaining such slab substantially level as it `is beinglifted.

fi abi l C Referring to the drawings in detail and more particularly first to Fig. l, the ground door of a building being erected is indicated by F, While the slab being lifted is shown at S.

Set into the ground door are a plurality of steel columns 1, 2, 3, etc. These are shown as in the nature of pipes, but might be of any other suitable shape. These columns extend to the full height of the building being erected.

Mounted on fthe top of each column is a supporting member 5, secured to the column as by means of an expansion bolt 6. Mounted on the supporting member 5 are a pair of hydraulic jacks '7 and 8, preferably arranged one above the other as shown, and provided with manual operating levers 7a and 8a. Each jack is also preferably equipped with a pressure gauge d.

Secured to the pistons of the jacks 7 and 8 are arms or force transmitting `members lll and ll. Fthese arms support lift rods l2 and 13. These rods are capable of freely rotating in the arms 1li and 1l and are shown `as provided at their upper ends with nuts 14 which bear against the arms to support the rods.

These rods l2 and 13 are threaded throughout the greater portion of their length, as best illustrated in Fig. 5, and work through nut blocks l5 set into the slab being lifted. That is to say, the rods are threaded throughout a length at least equal to the distance through which the slab is to be lifted. These nut blocks are shown as secured by screws i7 to shear collars i6 which surround the columns and preferably extend through the thickness of the-slab. l

it will be understood that an operator is stationed at each column to Work the jacks. When the jack 7, for example, is operated the `arm ill and rod 12 are moved upwardly, thus raising the slab a denite distance corresponding with the stroke of the jack. At the same time the rod 13 is raised with the slab and slides upward through the arm ll, so that the nut 14 no longer bears upon this arm but becomes spaced from it as shown in `Fig. l. After the jack 7 has completed its stroke, the operator then turns the rod i3 so as to screw it down through its nut block l5 until the nut id again engages the arm 1l. The operator then works -the handle Sa of the jack 8 and forces the arm ll upwardly, thus lifting the rod i3 and with it the slab S through a definite distance. As the jack 8 is operated the rod 12 is raised with the slab S and slides upwardly through the arm 16. Thereupon, after the jack 8 has completed its stroke, the operator turns the rod l2 so as to screw it down through its nut block, thus taking up the slack until the nut 14 again engages the arm lll.

It will thus be seen that, in lifting the slab, the jacks 7 and d are operated alternately, and that after the openation of each jack the lift rod ofthe other jack is screwed down through its nut block to take up the slack and to prepare for the next operation. It will be particularly noted that at the time that the lift rods are turned and are screwed down through their respective nut blocks they are unloaded. In other Words they turn freely, and at no time are the threads subjected to the load while 'oeing turned. In this way excessive wear of the threads is avoided.

It will be further understood that all of the operators stationed at the several columns operate their respective jacks in unison and it is of course important that the distance the slab is moved upwardly at each column should be the same as at all other columns, in order to avoid canting and binding of the slab. For this purpose` scale marks might be made on the columns, or separate scaled vertical rods at the various edges of the slab could be Aemploy-ed, in connection with pointers mounted on the umn.

slab. By watching these scales the operators could tell whether the slab is moving at each column at approximately the same rate. However, I have devised a much more efcient and accurate means for indicating whether the slab is maintained level, or in other words parallel to its original position. This means comprises a pair of cables 18 and 19 each of which is secured at one end to a movement measuring device such as a spring scale 22 suspended from an arm a of the supporting structure at each col- A bracket 21 is secured to the slab S adjacent 'each column and on each bracket are mounted a pair of pulleys 20. Each cable 18 and 19 passes around one of the pulleys on each bracket and thence passes freely through an opening 23 in the slab and is anchored at 24 to the ground iloor or another fixed support below the slab being lifted.

If the slab is of square shape as indicated in Fig. 2 it is preferable to provide two pairs of cables, one pair extending between columns 1 and 2, and the other pair extending between columns 3 and 4, the runs of these cables crossing each other at substantial right angles as indicated by the broken lines in Fig. 2 or as more clearly illustrated in Fig. 6.

From the foregoing it will be obvious that if all parts of the slab S move upwardly at the same rate the tension on all of these crossed cables will be the same and the pointers on the spring scales 22 will remain stationary.

They are preferably set so as to extend horizontally as shown in Fig. l. If, however, the left hand side, for example, of the slab S, as viewed in Fig. 1, should be lifted more than the right hand side, the tension on cable 19 would be increased, and this cable would actuate its spring scale and would pull the pointer downwardly. Similarly if the right hand side of the slab should be lifted more than the left hand side the tension on cable 18 would be increased and the pointer of the corresponding spring scale 22 would be displaced. Thus by watching the pointers of the spring scales, the operator at each 'column can be kept continuously informed as to whether or not the slab is being maintained substantially level. If it becomes canted or angularly displaced, steps can be taken to correct it.

In Figs. 3 and 4 I have illustrated slabs S' and S of diiferent shapes from that shown in Fig. 2. Fig. 3 is illustrated as being provided with six columns 1, 2, 3, 4, 2a, and 4'. With at slab of this shape it would be desirable to have the runs of the cables extend from column 1 to column 2 and from column 2 to column 4a, instead of crossing each other in the middle as in Fig. 2. In Fig.

`4 I have illustrated a slab provided with ten columns,

numbered as shown. In this case it would be desirable to have the runs of the cables extend from columns 1 to 4, and from columns 2 to 3b, as indicated by broken lines. In all case-s, whether the runs of the cables cross each other or not, it will be noted that they will be arranged at an angle to each other.

To illustrate the Versatility of the invention I have shown in Fig. 5 an installation in which two slabs S and S2 are arranged in laterally staggered relation. The slab S having been raised to its desired position on columns 1 and 2 is secured in position by collars 25 as shown.

The hoisting mechanism A (comprising the elements enclosed within the broken lines A in Fig. l) can be used to lift the slab S2, the lift rods 12 and 13 extending down through openings a in the slab S formed by removing the nut blocks 15 therefrom. These rods engage nut blocks in the slab S2 and operate as abovedescribed. The slab S2 is supported on columns 2 and 3 as shown, and the lifting operation is carried out in the same manner as already described in connection with Fig. l.

In Fig. 6 I have illustrated more in detail the construction of the actual hoisting apparatus which I prefer to employ. Instead of the arms 10 and 11 schematically illustrated in Fig. 1 I `have shown force transmitting members in the nature of crossheads 10' and 11' carried by the respective jacks 7 and 8. Four lift rods are erriployed, for better balance, two of them designated 12 and 12' being supported by the cross-head 11' and the other two, designated 13 being supported by the crosshead 10.

The jacks themselves are carried by a supporting structure comprising lower and upper base plates 26 and 27 connected `by struts 28. The lift rods have at their upper ends solid heads 14', in place of the nuts 14, and the rods may be turned by applying a wrench to these heads. At eachl operation of one of the jacks, the rods slide upwardly through the cross-head of the other jack, so that the heads 14 become spaced from the cross-head, as illustrated in connect-ion with the cross-head 10", and the rods may be screwed down to take up the slack, the same as in Fig. l.

The cable system for indicating any variation of the slab from a level position is substantially the same as that shown in Figs. l and 2, and comprises the crossed pairs of cables 18, 19, and 1S', 19', pulleys 20 being mounted on brackets 21 and the spring scales 22 being carried by arms 27B projecting from the upper base plates 27.

In Fig. 6, four columns 1, 2, 3, and 4 are illustrated. But, for the sake of clarity and to avoid confusion, lifting mechanism is illustrated in connection with only one column, namely, column 1.

In the erection of buildings by the lift slab method it may often be desirable to place the uppermost slab in a tilted or inclined position to constitute a sloping roof. In Figs. 7, 8 and 9 I have illustrated apparatus for accomplishing this.

Referring to these gures, I employ crossheads 10' and 11' generally similar to those shown in Fig. 6, but in order to permit rocking of these crossheads to some such position as that shown in Fig. 7 I support them on the jacks 7 and 8 by means of spherical bearings 29 and 30 received in similarly shaped sockets 31 and 32 formed in the crossheads.

The lift rods 12 and 12 4are supported on the crossheads by means of spherical Washers 33 working in similarly shaped sockets 34.

Similarly, in order to permit free tilting of the slab S, the lift rods work through spherical nuts 35 received in a similarly shaped socket member 36, held in place by a cover plate 37 through which pass bolts 38 into the shear collar 16. .In order to hold the spherical nut 35 against turning, while permitting free rocking movement, I provide it with a vertical slot 39 in which engages a stud 40.

Above the washers 33, each lift rod is provided with an elongated nut 41 screwed on to the upper threaded end of the rod. The nut 41 is provided near its upper end with a plurality of transverse holes 42 and the upper end of the rod is fonned with a slot 43. After the nut 41 has been adjusted as desired it may be locked to the nod by inserting a pin through this slot 43 `and a selected one of the holes 42.

The lower end of the nut 4l is formed with a series of splines 44 which loosely engage with mating splines 45 carried by a sprocket wheel 46 having at its upper end a hand wheel 47 and at its lower end a bearing 43 which is journaled in a split housing 49 held together by bolts 50. It will be seen that, by virtue of the above mentioned splines, the hand wheel 47 is locked to the nut 41 and the lift rod 12 so that the rod may be rotated by means of the hand wheel, and thus screwed down through the spherical nut block 35 as required.

Each pair of lift rods 12 and 12 is Provided with a sprocket Wheel 46 as above described, and a sp-rocket chain 51 passes around these two sprocket Wheels and gears them together so that both may be simultaneously rotated by means of the hand wheel 47. In this way both rods 12 and 12 are screwed down tln'ough their respective nut blocks at the `same rate. The same construction is of course employed in connection with the cross- S head .110. Other than `as above described the construction and operation of the lifting mechanism shown in Fig. 7 is the same as shown in Fig. 6, four lift rods being employed in connection with each column, in both cases.

In Figs. 8 and 9, B designates the mechanism enclosed within the broken line B of Fig. 7. In applying my improved level indicating means to a slab which is to be tilted to form a slopping roof as shown in Figs. 7 and 9, there would of course be no purpose in' having the cable runs extend between columns 1 and 2 and columns 3 and 4 as in Fig. 2. Instead I prefer to have the cable runs extend between 1 and 3 and between columns .2 and 4 as indicated at C in Fig. 8. This is for the reason that it is desirable to maintain the slab traversely level even though tilted longitudinally.

In Fig. 9 I have illustrated in broken lines the slab S in its original position on the ground floor F of the building, and have shown in full lines the position of this slab after it has been hoisted to an elevated and tilted ,position to constitute a sloping roof.

Although the manually operated jack arrangement so far described is desirable because of its light weight, simplicity, ease of set up and the absence of the need for supplying a suitable source of power, nevertheless, there is also need for power-operated hoisting equipment, v

especially in the construction of large groups of buildings where power is available and speed of operation is important. I have therefore devised a power-driven apparatus in which not only the manual operation of jacks has been eliminated but also the entire lifting operation is fully automatic, from start to finish.

This is illustrated diagrammatically in Fig. 10. In this ligure there is illustrated lifting mechanism associated Vwith only a single column, but it will be understood that this mechanism is duplicated for all of the other columns.

In this iigure, S designates the slab being lifted, as before, and this slab is provided with nut blocks 15 `through which extend the threaded lift rods 12 and 13".

Mounted on Ithe column 1 is a supporting structure 5', and this structure includes hydraulic jack cylinders 7" and 8". The pistons of these jacks are secured` to arms 52 and 53. The arm 52 supports an electric motor 54 coupled to a rotary pump 55. At the upper end of the lift rod 12 is a disc 56 disposed closely adjacent a similar disc 57 carried by the shaft of the motor 54, these discs constituting in effect a friction clutch.

Similarly the lift rod 13 carries at its upper end a disc 58 .adapted to cooperate with a similar disc 59 carried by the shaft of an electric motor 60, coupled to a rotary pump 61, both of which are supported by the arm 53.

62 and 63 designate reservoirs for containing motive iiuid. In van extension b of the supporting structure 5 are mounted a pair of oscillatory valves 64 and 65, which valves are oscillated by means of a walking beam 66 pivoted to the support at 67. This beam is in turn oscillated by a second walking beam 68 pivoted to the support at 69. This beam 68 is adapted to be engaged on either side of its pivot and oscillated by projections 52a and 53a formed on the arms 52 and 53.

Motive iluid is supplied from the reservoir 62 through pipe 70 to pump 55 and is delivered by this pump through a check valve 71 and pipe 74 to jack 8.

A pipe 72 connects check valve 71 with a port leading to valve 64, and a second port extends from this valve to pipe 73 leading back to the reservoir 62.

Similarly the pump 61 delivers uid from reservoir 63 through check valve '75 and pipe 76 to jack 7', and a pipe 78 connects the check valve 75 with a port leading to valve 65, the opposite port of which is connected by pipe 77 back to reservoir 63.

The apparatus shown in Fig. includes a spring scale 22 similar to those shown in the preceding figures, and

Vcontrolled by a cable in the same manner, but in this pase, in place of a pointer, the spring `scale has a movable contact 79 which is connected by wire 80 to `a slead 81. Associated with this is 'a Similar 1ead-82, these leads being connected by a switch 34 with the supply mains 83.

Current is supplied to the motor 54 by wire 85 tapping the lead 82, and the circuit is completed .by wire 87 which extends to a iixed contact 88. A similar tixed contact is mounted closely adjacent to contact 88 and is connected by wire 89 with the motor 60, the other side of this motor being connected by wire S6 with the lead 82.

A contact 91 is mounted at the end of walking beam 66 and is adapted to engageA the contacts 38 and 90. 'Ibis contact 91 is connected by a wire 92 with a iixed contact 93 positioned to be engaged by the contact 79 carried by the movable element of the spring scale 22. It will be understood that the leads 81 and 82 are extended as indicated at 94 to the hoisting mechanisms associated with all of the other columns used for lifting the slab, so `that all of these hoisting mechanisms operate similarly and in unison.

As shown in the drawing, the valve 65 `is closed andthe valve 64 is open. Also the contact 91 is in engagement with the contact 98. Furthermore the jack 7" is at the bottom of its stroke while the jack 8" is at the top of its stroke. When the switch 84 is closed therefore the motor 66 will be operated over a circuit including the wires 86 and 89, the contacts 90 `and 91 and the contacts 79 and 93. Since the valve 65 is closed, iiuid delivered by the pump 61 through check valve 75 will be forced to pass through pipe 76 to the bottom of jack 7", thus forcing this jack upward, raising the lift rod 12".

As the lift rod 12" pulls the slab upward, the slab carries along the lift rod 13" and thus brings the clutch discs 58 and 59 into contact. The rod 13 being then without load is rotated `by this clutch and is screwed down through its nut block 15. At the same time this screwing down of the rod 13 tends to force the arm 53 and piston of jack 8 downwardly.

Meanwhile, the valve 64 being open, iiuid is free to escape from jack 8 through pipe 74, check valve 71 and pipes 72 and 73 back to reservoir 62. As the jack 7" approaches the upper end of its stroke the projection 52a will tilt the beam `68 to dotted line position, thus swinging the beam `66. also to dotted line position and opening valve 65 and closing valve 64. At the same time the contact 91 shifts from contact 90 to contact 88, thus interrupting the circuit of motor 60 and establishing a circuit through motor 54.

When the beams 68 and 66 snap over into dotted line position as described the motor 54 is supplied with current and the pump 55 delivers motive iiuid through the check valve 71 to the jack 3, thus raising the lift rod 13. At the same time the lift rod 12, which is then unloaded, is moved upward with the slab thus bringing the clutch discs 56 and 57 into engagement thereby screwing the rod 12" down through its nut block 15. The upward movement of the jack 8 continues until the beams 6'8 and 66 are moved back into full line position, after which the cycle is repeated. This keeps up indefinitely until the slab has reached the desired position.

It will be noted that the supply of current to both motors passes through the contacts 79 and 93. If therefore at any time that portion of the slab adjacent column 1 is raised too far, relative to the other columns, the moveable contact 79, actuated by the spring scale 22, will be shifted downwardly out of engagement with contact 93, thus breaking the motor circuit and stopping the operation of the mechanism at column 1 until the mechanisms at the other columns have caught up. In other words, when the slab becomes level again the contact .79 will re-engage the contact 93 and normal operation Will be restored. By this means the slab is automatically maintained substantially level at all times.

If`it is desired to take care of either accidental or intentional tilting of the slab, spherical nut blocks, such as shown at 35 in Fig. 7, may be used in place of the 7 rigid nut blocks 15, and a larger clearance between the Slab and column can be provided. l

= It will be understood that my improved method of and apparatus Vfor the erection of buildings as described herein is intended for use primarily in connection with the lifting of slabs made of concrete and formed at ground level. My method contemplates pouring concrete on the ground floor as a form to produce a slab to be lifted to one of the upper oor positions. In some cases, a plurality of superposed slabs may be formed, one on top of the other, by interposing means between adjacent slabs to prevent the concrete being poured from bonding with the concrete of the slab below. In any event, however, the finished slabs are lifted successively by my lapparatus one at a time from the original position in which -they are formed, the slab first lifted being elevated to constitute the roof or uppermost oor, and secured permanently in position, and each of the successive slabs being lifted to the next lower lioror positions, and permanently secured to the columns.

What I, claim is:

l. In the artof hoisting pre-formed oor slabs in the erection of buildings, in which an upright fixed support is provided with a pair of hydraulic jacks mounted at the top of said support, and a different lift rod threaded throughout the greater portion of its length supported by each jack and working through a separate nut carried by the slab, the method which consists in operating said jacks alternately to raise said lift rods one at a time a definite distance at each operation, and while one of said jacks is being operated to raise one rod, screwing the other rod down through its nut, while not under load, to take up the slack.

2. In the art of hoisting pre-formed lioor slabs in the erection of buildings, in which an upright fixed support is provided with a pair of hydraulic jacks mounted at the top of said support, and a different lift rod threaded throughout the greater portion of its length supported by each jack-and working through a separate nut carried by the slab, the method which consists in operating said jacks alternately to raise said lift rods one at a time a definite distance, the raising of one rod serving to take the load off of the other rod, and while vone of said jacks is thus operated to raise one rod, screwing the other rod down through its nut, while not under load, as far as its jack will permit, to take up the slack.

3. The combination with an upright fixed support, of lifting mechanism carried by said support and comprising a pair of vertically spaced hydraulic jacks, a lift rod, threaded throughout the greater portion of its length, associated with each jack, means at the upper end of each rod by which it is rotatably suspended from its jack and by which it may be rotated, and nut blocks xed to an object to be lifted, through which nut blocks said rods work and through which said rods may travel throughout the length of their threaded portions.

4. In a lift slab construction, and in combination, hoisting means including an upright fixed support, a pair of hydraulic jacks mounted at the top of said support, nut blocks set in a slab to be lifted, and a vertically disposed lift rod associated with each jack, means by which each rod is rotatably supported on its jack and by which it may be rotated, said lift rods being threaded throughout a length at least equal to the distance through which the slab is to be lifted and working through said nut blocks, the stroke of each of said jacks being only a small fraction of such distance.

5. In a lift-slab construction, and in combination, hoisting means including an upright fixed support, a pair of vertically spaced hydraulic jacks mounted at the top of said support, a vertically disposed lift rod threaded throughout the greater portion of its length but being unthreadedly and freely rotatably supported on each jack laterally thereof, means at the upper end o f each rod by which it may be rotated, and nut blocks set in a slabv being hoisted, said threaded lift rods engaging said nut blocks, and through which nut blocks the said rods travel downwardly as they are rotated.

6. ln a lift-slab construction, and in combination, hoisting means including an upright fixed support, a pair of hydraulic jacks mounted one above the other at the top of said support, at least one lift rod capable of rotary and sliding movement with respect to each jack and threaded Vthroughout a substantial part of its length, said lift rod having a head fixed to its upper end, which serves to limit its sliding movement and by means of which head said rod is supported on the associated jack and may be rotated, and nut blocks fixed to a slab being hoisted, said threaded rods working through and extending substantially below said nut blocks, the working stroke of said jacks being only a small fraction of the distance through which the slab is to be lifted.

7. In a lift-slab construction, and in combination, hoisting means including an upright fixed support, a pair of hydraulic jacks mounted one above the other at the top of said support, a cross-head carried by each jack, a pair of threaded lift rods rotatably and unthreadedly connected with each crosshead, whereby there are four lift rods associated with said support, means at the upper end of each rod by which it may be rotated, and nut blocks set into a slab being hoisted, through which nut blocks said threaded rods extend a substantial distance on both sides and with which they engage.

8. Apparatus for the erection of buildings by lifting pre-formed oor slabs, such apparatus comprising an upright fixed support, a pair of hydraulic jacks mounted at the top of said support, a lift rod threaded throughout the greater portion of its length rotatably supported at its upper end by each jack, nut blocks set in a slab to be lifted, through which nut blocks said threaded lift rods work and through which they may travel throughout the length of their threaded portion, means by which each rod may be screwed downwardly through its nut block relative to its associated jack, and means for independently operating each jackto raise its rod and` thus lift said slab.

Y 9. In a lift-slab building construction, the combination of a generally horizontally extending, fixed base; a preformed slab to be lifted to a position spaced above said base, said slab initially resting on said base and provided with a plurality of openings; a pair of laterally spaced, xed, vertical columns each extending freely through a different one of said openings and each having an upper end portion disposed above the position to which said slab is to be lifted; a pair of vertically extending lift rods disposed adjacent each of said columns, said lift rods each extending from a point adjacent said base to a point above v the position to which said slab is to be lifited and adjacent the upper end portion of the corresponding one of said columns and being continuously threaded from its lower end to a point at least as high as the position to which said slab is to be lifted; a pair of jacks rigidly mounted on the upper end portion of each of said columns, each jack of said pair of jacks including a vertically movable force-transmitting member, leach lift rod of one of said pairs of lift rods being connected to a different one of said force-transmitting members, whereby selective operation of the jacks of said pair of jacks will selectively raise the lift rods of said pair of lift rods; said lift rods extending a substantial distance above and below said slab and being rotatable and upwardly slideable with respect to the force-transmitting members to which they are connected; said jacks all being constructed and arranged to move their respective force-transmitting members at each stroke through a vertical distance which is a small fraction of the length of said lift rods; a plurality of nut blocks secured to said slab, each of said lift rods being threadably engaged in and working through a different one of said nut blocks, whereby upward movement of any of said force-transmitting members byactuation of the corresponding one of said jacks will impart an incremental lifting movement to said slab` by means of the corresponding one of said lift rods, and rotation of any of said lift rods, at a time when the jack associated therewith is not operated to impart such lifting movement, will cause such lift rod to travel downwardly through said slab; means by which said lift rods can be individually rotated, and means whereby the jacks of each pair of jacks may be operated alternately.

10. In the erection of buildings, the combination with apparatus for lifting pre-formed slabs up to their desired positions and including a plurality of fixed spaced columns and hoisting mechanism mounted at the top of each column and operatively connected with a slab to be lifted, of means for indicating any angular displacement of said slab as it is being lifted, said means including a movement measuring device carried by each of two spaced columns, a pair of pulleys mounted on the upper side of `said slab adjacent each of said two columns, and a pair of cables, ,each cable passing around one pulley of each pair of pulleys and attached at one end to one of said movement measuring devices and anchored at the other end to a fixed support.

l1. In the erection of buildings, the combination with apparatus for lifting pre-formed slabs up to their desired positions and including a plurality of fixed spaced columns and hoisting mechanism mounted at the top of each column and operatively connected with a slab to be lifted, of means for indicating any angular displacement of the slab as it is being lifted, said means including a movement measuring device carried by each of two spaced columns, two pairs of pulleys mounted on the upper side of said slab one pair adjacent each of said two spaced columns, and a pair of cables each passing around one of each of said pairs of pulleys, each of said cables being attached at one end to a different one of said movement measuring device and anchored at its other end to a fixed support.

12. In the erection of buildings, the combination with apparatus for lifting pre-formed slabs up to their desired positions and including a plurality of fixed spaced columns and hoisting mechanism mounted at the top of cach column and operatively connected with a slab to be lifted, of means for indicating any angular displacement of the slab as it is being lifted said means including a movemeut measuring device carried by each of a plurality of said spaced columns, a plurality of pairs of pulleys mounted on the upper side of said slab, one pair adjacent each of said columns, and two pairs of cables having runs substantially parallel with said slab but disposed at an angle to each other, each cable of each pair passing around one pulley of each of two of said pairs of pulleys and being connected at one end with one of said movement measuring device and anchored at its other end to a fixed support independent of and at a point below said slab, said slab having openings adjacent said pairs of pulleysthrough which said cables freely pass.

13. The combination with a fixed support and an object to be lifted, of hoisting mechanism mounted on said support and comprising two separate electric motors, two Atacks, each operated by one of said motors, a threaded lift rod supported by each jack, nut blocks carried by the ob ject being lifted and with which nut blocks said threaded lift rods engage, means for supplying current to said motors, automatic means whereby said jacks are alternately operated to move said lift rods upwardly a dente distance te at each operation, and means whereby, during the interval between successive operations of each jack, the lift rod of the other jack is automatically screwed down by its associated electric motor through its nut block to take up the slack.

14. Apparatus for the erection of buildings by lifting pre-formed floor slabs, the combination with a plurality of fixed supports, of hoisting mechanism mounted at the top of each support and comprising two separate electric motors, a pump driven by each motor, two hydraulic jacks each supplied with motive fluid by one of said pumps, a threaded lift rod supported by each jack, nut blocks carried by a slab being lifted and with which said threaded lift rods engage, means for supplying current to said motors, automatic means whereby said jacks are alternately operated to move said lift rods upwardly, and means whereby, during the interval between sucessive operations of each jack, the lift rod of the unloaded jack is automatically screwed down by its associated electric motor through its nut block to take up the slack,

15. Apparatus for the erection of buildings by lifting pre-formed oor slabs, the combination with a plurality of fixed supports, of hoisting mechanism mounted at the top of each support, and including two hydraulic jacks, a threaded lift rod suported by each jack, nut blocks carried by a slab being lifted and through which said threaded lift rods work, automatic means whereby said jacks are successively alternately operated to move said lift rods up`- wardly one at a time a predetermined distance, and means whereby, during the intervals between successive operations of each jack, the lift rod of the unloaded jack is automatically screwed down through its nut block to take up the slack.

16. Apparatus for the erection of buildings by lifting pre-formed floor slabs, the combination with a plurality of fixed supports, of hoisting mechanism mounted at the top of each support and including two separate electric motors and a threaded lift rod associated with each motor, nut blocks carried by a slab being lifted and through which said threaded lift rods work, means whereby said lift rods are alternately raised a predetermined distance, and means whereby, when one life rod is being raised, so as to take the load off of the other, the unloaded rod is automatically screwed down by its associated electric motor through its nut block to take up the slack.

References Cited in the file of this patent UNITED STATES PATENTS 216,326 Hyde June 10, 1879 1,066,436 Peltzer July 1, 1913 1,446,774 Townsend Feb. 27, 1923 A1,539,491 Ionides May 26, 1925 1,710,442 Warshaw Apr. 23, 1929 2,102,008 Klitsche Dec. 14, 1937 2,503,659 Curtis Apr. 11, 1950 2,592,634 Wilson a Apr. 15, 1952 2,686,420 Youtz Aug. 17, 1954 2,715,013 Slick Aug. 9, 1955 2,758,467 Brown et al Aug. 14, 1956 2,785,809 Roblet Mar. 19, 1957 f 2,867,111 Youtz Ian. 6, 1959 FOREIGN PATENTS 600,462 Great Britain of 1948 OTHER REFERENCES Engineering News Record, page 58, Aug. 12,1954. 

